External and internal cloud condensation nuclei (CCN) mixtures: controlled laboratory studies of varying mixing states

Vu, Diep; Gao, Shaokai; Berte, Tyler; Kacarab, Mary; Yao, Qi; Vafai, Kambiz; Asa-Awuku, Akua

doi:https://doi.org/10.5194/amt-12-4277-2019

Articles | Volume 12, issue 8

https://doi.org/10.5194/amt-12-4277-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/amt-12-4277-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 12, issue 8

Research article

|

08 Aug 2019

Research article |

| 08 Aug 2019

External and internal cloud condensation nuclei (CCN) mixtures: controlled laboratory studies of varying mixing states

Diep Vu, Shaokai Gao, Tyler Berte, Mary Kacarab, Qi Yao, Kambiz Vafai, and Akua Asa-Awuku

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Final revised paper (published on 08 Aug 2019)
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Preprint (discussion started on 11 Jan 2019)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Reviewer comment', Anonymous Referee #2, 16 Feb 2019
- AC1: 'Response to Referee #2', Akua Asa-Awuku, 10 May 2019
RC2: 'Review report on Vu et al.', Anonymous Referee #3, 15 Apr 2019
- AC2: 'Response to Referee #3', Akua Asa-Awuku, 10 May 2019

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Akua Asa-Awuku on behalf of the Authors (10 May 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (15 May 2019) by Mingjin Tang

RR by Anonymous Referee #4 (28 May 2019)

Suggestions for revision or reasons for rejection

Vu et al. present measurements of CCN activity for externally and internally mixed particles. The subject is of relevance to study, since several studies report a significant sensitivity to the mixing state of CCN aerosol in atmospheric models.
The manuscript has been submitted to a technical journal, but there is little focus on an appropriate and detailed characterization of any technical advances. The choice of characterizing the experimental system mainly by CCN spectra does not seem appropriate, and the interpretation of results can in some cases not be justified by the data presented. The concluding statements appear trivial and are not novel – which in principle is acceptable for a technical paper. However, as described above, there is not a strong focus on the technical aspects in this manuscript.
In general, the manuscript is not well written – there are very many examples of unclear formulations. There is a lack of many basic details regarding the experimental approach. Several of the choices made and the argumentation (and lack of argumentation) presented by the authors appear questionable.
I recommend that this manuscript is rejected.
Below, only a few examples of major issues are described. Addressing those issues alone would not qualify the manuscript for publication.

One major issue with this study is that succinic acid particles are used to create internally and externally mixed particles. Succinic acid (SA) is known to evaporate from hydrated aerosol particles (Riipinen et al., 2006) as well as from dry aerosol particles at room temperature (Wex et al., 2007). Hence, the choice of using succinic acid in aerosol mixtures for CCN studies introduces significant issues, which turn further pronounced when a heating step is applied.
Vu et al. claim that ‘very’ dry SA particles remain externally mixed with other aerosol particles until a preheating step is removed resulting in dry SA particles – and then homogeneous internally mixed particles occur in their flow tube. The authors do not present a reasonable explanation for such unexpected behavior. However, it is clear that this interpretation cannot be justified from the data presented in Fig. 9. As long as the pre-heating step is applied, the SA particles seem to be dominated by a mode centered near a mobility diameter of ~20 nm (e.g. scan 54). When the preheating is turned off, then the resulting particle number size distribution has a clear maximum near 80-85 nm decreasing significantly towards 130 nm (e.g. scan 73), where the soot particle size distribution supposedly has a maximum. A reasonable explanation for the behavior is a transition from very pronounced evaporation of SA particles, to a somewhat lower evaporation of SA particles, which leads to the observed transition in the CCN spectra and dominance of SA in the size range studied.
Hence, the interpretation of the behavior of the experimental system presented does not seem appropriate. It would be meaningful to characterize the system with more well-behaved substances and also with additional instrumentation more sensitive to the mixing state than a CCNC setup. Furthermore, the system should be characterized with more substances, if indeed the system behavior is very sensitive to the aerosol RH, as claimed by the authors.
The issues described above also influence the interpretation and use of results presented for internally mixed SA-NaCl particles as well as the externally mixed SA-ammonium sulfate particles.

References
Riipinen et al., Atmos. Res., 82, 579-590, 2006.
Wex et al., Geophys. Res. Lett., 34, Li7810, 2007.

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RR by Anonymous Referee #3 (29 May 2019)

RR by Anonymous Referee #5 (23 Jun 2019)

ED: Publish subject to minor revisions (review by editor) (23 Jun 2019) by Mingjin Tang

AR by Anna Mirena Feist-Polner on behalf of the Authors (04 Jul 2019) Author's response

ED: Publish as is (06 Jul 2019) by Mingjin Tang

Short summary

Aerosol–cloud interactions contribute the greatest uncertainty to cloud formation. Aerosol composition is complex and can change quickly in the atmosphere. In this work, we recreate a transition in aerosol mixing state in the laboratory, which (to date) has only been observed in the ambient state. We then report the subsequent changes on cloud condensation nuclei (CCN) activation.